1. Field of the Invention
The present invention relates to implantable electronic medical devices, such as cardiac pacemakers and defibrillators for example, for stimulating tissue of animal for the therapeutic purposes; and more particularly to electrical leads for such devices.
2. Description of the Related Art
Numerous medical conditions, such as cardiac and neurological dysfunctions, are treated by an implanted electronic device, which provides electrical stimulation to the affected tissue of the animal. These devices have a plurality of metal components, including the case enclosing electronic circuits and wire leads extending from the case to electrodes in contact with the tissue to be stimulated or monitored.
Magnetic resonance imaging (MRI) is commonly employed to view internal organs of medical patients. To create an image, the patient is placed into very strong static and varying magnetic (gradient) and radio frequency (RF) fields and thus MRI generally is prohibited for patients with implanted ferromagnetic and or electrically conductive objects. Although it is feasible to minimize and even eliminate the use of ferromagnetic materials in implanted apparatus, electronic devices, such as cardiac pacemakers and defibrillators, require electrically conductive components and lead structures that are affected by the fields produced by an MRI scanner.
It has been a long-standing goal to make implanted devices MRI compatible so that this imaging modality can be used with patients having those devices. There are several reasons for achieving this goal. Firstly, incompatible implant components and leads induce susceptibility difference, which destroys DC magnetic field homogeneity of the MRI scanner, thereby negatively affecting the imaging performance. Secondly, the MRI field can produce eddy currents in the implanted conductive materials, which currents generate heat that adversely affects patient and degrade the scanner performance by field distortion. Thirdly, the MRI RF, gradient and magnetic fields may ruin the implanted device. Fourthly, the incompatible implant material can potentially cause serious internal injuries to the patient.
Typical electrical leads used with implanted medical devices had a proximal end connected to the electronic circuit inside the main case of the device and a distal end having an external electrode to contact the tissue of the animal being stimulated. The connection of the conductor in the lead to the external electrode also is important to the proper functioning of the implanted medical device. Good electrical and mechanical connection must be established.
Previously the lead conductor was attached by an adhesive that bonded to one or more grooves in a conductive ring member that served as the electrode.
In other places, a separate connector was used to interconnect two conductors. Here the connector has an electrically conductive body with a first end portion coupled to one conductor and a second end portion coupled to a the other conductor. In one example, electrically conductive connector had one or more internal grooves (or threads) to which the conductor was coupled. Conductors also were secured to the electrically conductive connector by rotary swaging, laser or resistance welding, brazing, mechanical swaging, or crimping. In examples in which one or both of first conductor or second conductor are coupled via one or more external grooves (such as those associated with screw threads), shrink tubing or a compressive/elastic lead body may be used to further secure such conductors to connector.
The conductor of an electrical lead may also be coupled to a ring member using a variety of techniques. One technique includes a securing member disposed around the distal end portion of the conductor and the ring member. Optionally, one or more grooves or threads may be formed on the ring member and the securing member is deformed over the conductor thereby making connection the ring member. In a first technique, portions of the securing member are pushed into or over the one or more grooves or threads. A second technique uses a conductive adhesive to couple the conductor to the ring member. A third coupling technique involves forming one or more grooves or threads on the ring member and urging the conductor onto the one or more grooves or threads. Such urging may come by way of the compressive nature of the lead body or a removable preformed mandrel.
In any case, lead-electrodes come in a variety of configurations including tip and ring for pacing and coiled configuration for implantable cardiac defibrillator (ICD) applications. The electrode structure and lead-electrode interface must be selected to minimize the build up of induced local electrical fields, which can give rise to radio frequency burns and tissue damage. In general, the lead-electrode material choices affect image quality and MRI compatibility. The lead-electrode interface needs to be mechanically fatigue resistant, yet electrically conductive, and bio-compatible.
Therefore, there is a desire to provide an electrode structure and a lead-electrode interface that satisfies the above requirements. In addition, it is desired that the interface has minimal complexity for ease of manufacture.